I guess you haven't noticed the total redesign of notifications, widgets and banner/alerts? Or the Music app or the News app? Again...just because you may not like the changes or think there jasnt been a lot, or in your words "a lack of". Doesn't mean there hasn't been any or there are less of them.
Right, the "dark mode" of OS X/macOS that literally only changes the dock and menu bar. Or the "dark mode" of tvOS that only changes the white home screen to grey. Those are not even close to a true dark mode and barely count.
True, OLED is only a rumor. But far more likely than the old sapphire screen or liquid metal shells rumors. It's going to happen because it has big benefits in a mobile device like an iPhone. And the major benefit is a true system wide dark mode, because blacks on OLED are off pixels. Which saves battery drain in a device that battery life matters a lot. That is why it doesn't make sense in an iPhone until the inevitable switch to OLED. That is why watchOS is mostly black. To save as much battery possible with its tiny battery, optimizing one of the biggest OLED advantages.